The present invention relates to a color electrophotographic apparatus applicable to color printers, color copying machines, and color facsimiles.
In a color electrophotographic apparatus, a full color image is formed on an acceptor sheet such as a paper sheet by superimposing images of black, yellow, magenta and cyan toners. The color electrophotographic apparatus includes a continuous transfer type using a transfer belt in an image superimposing member for forming a full color image, and a transfer drum type using a transfer drum in the image superimposing member.
A conventional color electrophotographic apparatus disclosed in the Japanese Patent Application Laid Open Hei No. 7-36246 will be explained with reference to FIG. 25.
FIG. 25 is a side cross-sectional schematic view showing an entire configuration of a conventional color electrophotographic apparatus. The conventional color electrophotographic apparatus shown in FIG. 25 is the continuous transfer type using an intermediate transfer belt in the image superimposing member, a sheet up to an A-4 paper size (210.times.297 mm) can be printed. In the below-mentioned explanation, the same numeral will be applied to portions having the same function, and in the case that the toner color needs to be differentiated, letters designating the colors such as Y for yellow, M for magenta, C for cyan, and BK for black are added to the numerals.
In FIG. 25, an intermediate transfer belt unit 101 comprises an intermediate transfer belt 102, a first transfer roller 103, a second transfer roller 104, a cleaner 105, and a waste toner box 106. In the configuration, the full color image is formed by superimposing toner images of the four colors on the intermediate transfer belt 102.
As shown in FIG. 25, the conventional color electrophotographic apparatus comprises four sets of image forming units 107BK, 107C, 107M, 107Y of the same shape for black, cyan, magenta and yellow toners. The image forming units 107BK, 107C, 107M, 107Y are arranged in a substantially ring shape to form a substantially columnar assembly of image forming units 108. Each of the image forming units 107BK, 107C, 107M, 107Y is mounted in a predetermined position in the color electrophotographic apparatus. Thereby, the image forming units 107BK, 107C, 107M, 107Y are coupled to a driving system and an electric system of the conventional color electrophotographic apparatus by a coupling member (not shown), so that the image forming units 107BK, 107C, 107M, 107Y are integrated mechanically and electrically.
In the assembly of the image forming units 108, each of the image forming units 107BK, 107C, 107M, 107Y is supported by a supporting member (not shown) to be arranged in a ring-like shape. The assembly of the image forming units 108 is driven by a driving motor (not shown), so that the assembly of the image forming units 108 is rotated around a cylindrical shaft 109 arranged at the center of the assembly of the image forming units 108. The cylindrical shaft 109 is unrotatably fixed to an outer case of the conventional color electrophotographic apparatus.
When each of the toner images is formed, the image forming units 107BK, 107C, 107M, 107Y are rotated so that the respective photoconductors 118 are located at a image forming position 110 facing to the first transfer roller 103 for supporting the intermediate transfer belt 102. The image forming position 110 is an exposed position where laser signals 111 from a laser exposure device 112 expose the photoconductors 118.
As shown in FIG. 25, the laser exposure device 112 is arranged at a lower side of the conventional color electrophotographic apparatus. As shown by a dashed line in FIG. 25, the laser signals 111 irradiated from the laser exposure device 112 reach an opening formed on the shaft 109 through an entrance 113 of an optical path between the image forming unit 107M for magenta and the image forming unit 107C for cyan. The laser signals 111 are reflected by a mirror 114 fixed in the shaft 109. The laser signals 111 reflected by the mirror 114 enter the image forming unit 107BK located at the image forming position 110 through an exposure entrance 115 of the image forming unit 107BK.
The laser signals 111 having entered the image forming unit 107BK irradiate a side portion to be exposed of the photoconductor 118 through an optical path between a developing unit 116 and a cleaner portion 117. The laser signals 111 irradiating the photoconductor 118 are scanned in a direction of a generating line on the photoconductor 118, so that the photoconductor 118 is exposed and a black toner image is formed thereon.
The black toner image formed on the photoconductor 118 is transferred to the intermediate transfer belt 102 by the contact with the intermediate transfer belt 102.
Subsequently, the assembly of the image forming units 108 is rotated in the clockwise direction (shown by an arrow "Y") by 90 degrees, so that the image forming unit 107Y for yellow is arranged at the image forming position 110. Operations similar to that of the above-mentioned forming process for the black toner image are conducted, and thereby, a yellow toner image is superimposed on the black toner image transferred on the intermediate transfer belt 102.
Operations similar to that of the above-mentioned forming process for the black toner image or the yellow toner image are conducted in the image forming units 107M and 107C for magenta and cyan. Thereby, the full color image is formed on the intermediate transfer belt 102.
The full color image on the intermediate transfer belt 102 is transferred on a paper sheet supplied to the conventional color electrophotographic apparatus by a third transfer roller 119, and further the full color image transferred on the paper sheet is fixed by a fuser 120.
In the conventional color electrophotographic apparatus, four developing units 116 for each color are formed by the same size based on the premise that the black, cyan, magenta and yellow toners are consumed at the same rate.
However, in the inventor's experimental investigation, it is found that each color has a different consumption amount, and in particular, a black toner is consumed in a greater amount compared with the other colors. Therefore, in the conventional color electrophotographic apparatus, there occurs problems in that the black toner needs to be supplied more frequently than the other colors, requiring labor in the maintenance.
Furthermore, in the conventional continuous transfer type color electrophotographic apparatus, each of the four photoconductors 118 comes in contact with the intermediate transfer belt 102 and comes apart from the intermediate transfer belt 102 repeatedly. Therefore, there is a problem in that the intermediate transfer belt 102 stretched with a high tension is liable to generate vibration, resulting in disturbing the image. In particular, in the apparatus having a long distance between the roller shafts for stretching the intermediate transfer belt 102, an amplitude of the vibration becomes larger.